1. Field of the Invention
The present invention relates to the field of semiconductor devices, and more specifically, to a process for decreasing outdiffusion of fluorocarbons from low k materials.
2. Background Information
Integrated circuits manufactured today are made up of literally millions of active devices such as transistors and capacitors formed in a semiconductor substrate. These active devices are formed and interconnected in an elaborate system of layers. As semiconductor devices become smaller and faster there are certain limiting factors that must be overcome for next generation devices.
One such factor is the RC Time Constant. The RC Time Constant is the fundamental limit of a microprocessor caused by the capacitance between the metal lines of the microprocessor. There are two things which determine the RC Time Constant: the resistance of the metal lines themselves and the capacitance of the dielectric materials. The most commonly used dielectric material is silicon dioxide (SiO.sub.2), which has a dielectric constant (k) of approximately k=4. Thus, in order to reduce the RC Time Constant many manufacturers are turning to materials with lower dielectric constants.
One way which has been found to reduce the dielectric constant of oxides, such as silicon dioxide, is to dope the oxide. For example, doping silicon dioxide with fluorine reduces the dipole of silicon dioxide and thereby reduces the dielectric constant. Using fluorine to dope an oxide creates what is called a fluorinated silicate glass (FSG or SiOF). FSG has a dielectric constant of approximately k=3.3-3.7.
Another way to reduce the dielectric constant is to replace the oxide with another low k material altogether, for example low k polymers. Low k polymers have been found to have dielectric constants in the range of approximately k=1.8-3.3. Some examples of low k polymers are polyaromatic ethers (PAEs), aerogels, xerogels, and parylene.
By using low k materials several advantages may be obtained. The use of low k materials have been found reduce the RC Time Constant due to a decreased capacitance. Reducing the RC Time Constant helps to increase the speed of the device. The use of low k materials have also been found to improve power dissipation and to reduce crosstalk noise between metal lines.
Unfortunately, low k materials tend to be made up of fluorocarbons and have a lower thermal stability than previously used oxides. During the process sequence to fabricate transistors and their interconnect structures temperatures in excess of 400.degree. C. are often reached. As the processing temperatures climb and the low k materials heat up, fluorine and other chemical species from the low k materials (due to their lower thermal stability) tend to outdiffuse into the surrounding metal layers. Fluorine outdiffusion is known to cause corrosion in the metal layers and to create potential adhesion problems with juxtaposed dielectric films which can ultimately create yield and reliability problems in the devices being fabricated.
Thus, what is needed is a method to decrease fluorine outdiffusion from low k polymers during higher temperature processing.